1. Field of the Invention
This invention relates to thermoforming equipment for differentially pressure forming an article in a sheet of thermoplastic material and more particularly, to apparatus for preheating the sheet prior to its entry into a differential pressure thermoforming machine.
2. Description of the Prior Art and Objects
Differential pressure forming apparatus, such as that disclosed in U.S. Pat. No. 3,346,923 issued to G. W. Brown, et al, on Oct. 17, 1967 and U.S. Pat. No. 3,664,791, issued to G. W. Brown on May 23, 1972, which are incorporated herein by reference as though fully set forth herein, have been provided heretofore for successively delivering a heated thermoplastic sheet to an oven at a sheet heating station and then to a forming station at which molds engage the sheet to differentially pressure form an article in the sheet.
In these prior art machines, the thermoplastic sheet is typically laterally captured by impaling the lateral edges of the sheet on sheet carrying chains which carry the sheet through the differential pressure forming machine while the articles are being formed therein. Such machines typically include ovens which heat the sheet prior to applying differential pressure to the sheet at a forming station. It has been found that, with some plastics, when the sheets are initially heated, they stretch and twist. If the plastic sheets are laterally captured, wrinkles sometimes develop in the sheets which obviously cause formation problems resulting in defects in the product being formed.
Toward this end, preheaters have been provided heretofore for heating the thermoplastic sheet prior to its entry into the oven of a thermoforming machine.
One such preheater has included a plurality of heated rolls around which the sheet is successively turned or wrapped. A sufficient number of rolls are utilized to preheat the sheet to a desired temperature prior to its entry into the thermoformer or oven. While passing through the preheater, the sheet is not laterally restricted. The problems attendant to this prior art construction, is that the sheet heating rolls are rather expensive and it is inconvenient to successively thread the sheet in a seriatim fashion through and around the rolls. Accordingly, it is an object of the present invention to provide a new and novel preheater machine for preheating a thermoplastic sheet prior to its entry into a thermoformer oven.
It is another object of the present invention to provide a preheater for reducing the cost and increasing the convenience of preheating the thermoplastic sheet.
Another type of preheater is sold by the Hemmerle Corporation under the trademark or model no. PP-BH23 which includes a vertically disposed oven that provides a vertical tunnel through which a thermoplastic sheet passes before entering the thermoforming machine. The sheet is fed in a horizontal path into the machine through an inlet and thence downwardly where the sheet is trained around a set of lower rollers and thence upwardly through the vertical oven and thence over top rollers and then downwardly through a discharge opening. A serious problem attendant to this machine is the inconvenience and time required to thread the sheet around the vertically and horizontally spaced apart rollers and pass the sheet through the preheater oven chamber. The preheater oven is initially laterally moved out of the path of the sheet until the threading is complete and then the oven is returned to a sheet heating position adjacent the sheet. Accordingly, it is another object of the present invention to provide a preheater which will eliminate the disadvantages of the prior art machines.
Still another of the present invention is to provide a preheater for preheating a thermoplastic sheet which will increase the speed and efficiency of operation.
Another object of the present invention is to provide a preheater of the type described which includes sheet displacing mechanism which will minimize the time required to thread the thermoplastic sheet through the pre-heater.
A further object of the present invention is to provide a method of preheating and a preheater of the type described including sheet displacement mechanism for threading the sheet through a preheating station, in a horizontal path, concurrently vertically displacing upstream and downstream portions of the sheet at the preheating station in vertically opposite directions to provide a vertical sheet portion which is laterally aligned with a vertical passage provided in a vertical sheet heating oven.
A still further object of the present invention is to provide a preheater and method of preheating of the type described which will allow the thermoplastic sheet to be initially disposed in a horizontal plane at the sheet heating station and then vertically moving upstream and downstream portions of the sheet in different vertical directions while the remainder of the sheet remains horizontal to provide a vertical sheet portion which is received by a vertical oven.
Another object of the present invention is to provide a preheater for preheating a thermoplastic sheet of the type described including apparatus, having inlet and outlet rollers, for initially disposing the sheet in a horizontal plane at a sheet heating station, vertically displacing upstream and downstream portions of the sheet at the sheet heating station between the inlet and outlet rollers to vertically spaced apart positions to provide an upstanding sheet portion, apparatus for laterally moving a vertical oven between a laterally removed position and a sheet heating position vertically adjacent the upstanding sheet portion, and apparatus for moving the sheet in a downstream path.
A further object of the present invention is to provide a preheater of the type described which includes inlet and outlet rollers which grip inlet and outlet portions of the sheet and hold the sheet while portions of the sheet between the inlet and outlet rollers are vertically displaced to provide a vertical sheet portion which is heated by an upstanding vertical oven.
It is a still further object of the present invention to provide new and novel preheating mechanism, of the type described including nip forming rolls which are moveable between relaxed positions allowing a thermoplastic sheet to be threaded therethrough and nip forming positions holding inlet and outlet portions of the sheet.
It is another object of the present invention to provide preheating mechanism of the type described which includes an outlet nip forming roll which will grip and clamp the downstream portion of the sheet to preclude movement of the sheet in an upstream direction when portions of the sheet are moved vertically out of the plane of the sheet to provide a vertical sheet portion that is received by a vertical oven.
It is another object of the present invention to provide a preheater of the type described including new and novel carriage mechanism for concurrently moving downstream and upstream portions of the sheet in vertically opposite directions to provide a vertical sheet portion which is disposed in a vertical position receivable by a laterally moveable oven.
Another object of the present invention is to provide a method of preheating a sheet including the steps of threading a continuous sheet in a first plane, transversely concurrently displacing upstream and downstream portions of the sheet in opposite directions relative to the first plane to dispose a transverse portion of the sheet between the upstream and downstream sheet portions in a second plane transverse to the first plane, and thereafter, moving the sheet in a downstream path of travel and heating the transverse portion of the sheet.
Other objects and advantages of the present invention will become apparent to those of ordinary skill in the art as the description thereof proceeds.